1. Field of the Invention
The present invention relates to an electrophotographic toner and an electrophotographic developer that can be preferably used when forming an invisible image together with a visible image on the surface of an image output medium such as recording paper and also relates to an image formation method using these toner and developer.
2. Description of the Related Art
Conventionally, there are attached data embedding technologies for superimposing and embedding attached information in an image. In recent years, utilization of these attached data embedding technologies has been increased, especially for copyright protection for products such as digital books and their still pictures, and for the prevention of illegal copying of these digital books.
When using the attached data embedding technologies for digital books, image data in which attached data such as a copyright ID and a user ID have been embedded are circulated. The data is embedded in such a manner so as to be visually unnoticeable. Diverse measures are incorporated into color image forming devices in order to prevent the forgery of securities and the like. One of these measures includes technologies for superimposing a symbol, which is difficult to visually discern on an image and is unique to the image forming device. The symbol is superimposed on the image data via fixed gradation. This is for identifying the image forming devices used for copying and prnting.
When using these technologies, even if securities are forged using an image forming device, the image of the forged product can be read by a reader capable of extracting a specific wavelength region, so that the symbol unique to the image forming device could be deciphered. Therefore, the image forming device used for forging is identified by deciphering this symbol and an effective clue can be obtained to aid in the capture of the forger.
However, the above-mentioned technologies have several problem. Namely, even if a symbol inherent to an image forming device is superimposed in a low density range, it is not reflected on the image density. Hence, the symbol cannot be read. Also, the superimposed symbol inherent to the image forming device can be easily identified by the eye in a density range with high gradation contrast, depending on the gradation characteristics of the image forming device.
Given the situation, various technologies has been taught, for example, the technologies described in Japanese Patent Application Laid-Open (JP-A) Nos. 1-225978, 6-113115, 6-171198 and 6-122266. These well-known technologies for embedding attached information in such a manner so as to be visually unnoticeable.
The technologies described in JP-A No. 1-225978 are for forming an invisible image by forming an electrostatic latent image corresponding to image information on a latent image support, developing this electrostatic latent image by using an insulation toner having a polarity inverse to that of the electrostatic latent image, and high transparency, to form an invisible toner image. Transferring and fixing the invisible toner image to a transfer material is carried out. The visualization of the invisible image obtained in this manner is accomplished by charging only the insulation toner portion on the transfer material and by developing the portion using a color toner.
In the technologies described in JP-A No. 6-113115, pattern forming devices differing from each other in an image forming system are provided separately to record a given pattern by using a recording material having a characteristic peak of spectral reflection in a wavelength range from 450 nm or less and 650 nm or more.
The technologies described in each of JP-A Nos. 6-171198 and 6-122266 are as follows. Specifically, a color region comprising an infrared absorbing dye and a color region comprising an infrared reflecting dye are formed in parallel or in an overlapped manner on a substrate by using an electrophotographic system, electrostatic recording system or ink jet recording system, to form an image such that at least one of the color regions is used to form an image such as characters, numerals, symbols and patterns and the above two color regions are not substantially discriminable or distinguishable with difficulty by naked eyes.
Also, an image formation method having the same concept as above is described in JP-A No. 2001-265181, which, however, does not refer to an electrophotographic toner in detail.
In the meantime, as image forming materials for forming an invisible image by using materials absorbing near-infrared light, methods utilizing materials containing rare earth metals such as ytterbium are proposed in each of JP-A Nos. 9-104857 and 9-77507. Also, in JP-A No. 7-53945, a method of utilizing an infrared absorbing material containing copper phosphoric acid crystallized glass is proposed.
However, there are the following problems in the conventional technologies described in the above publications. Specifically, the technologies described in JP-A No. 1-225978 have the drawback that when reading the attached information which is the invisible image, a color toner is developed only on the invisible toner portion of the image to visualize the image and therefore the document is denatured once the image is visualized, with the result that after the image is visualized, the image cannot be utilized as a document in which an invisible attached information is embedded.
Also, in the technologies described in JP-A No. 6-113115, nothing is defined concerning the absorptivity of the recording material in the visible region. Therefore, there is the case where it is necessary to dispose a shielding layer for visually shielding the information as the upper layer on the region where the attached information is embedded. Namely, there is the case where the problem arises that the region and image in which the attached information is embedded are limited. Usually, a shielding layer for shielding information visually must absorb or reflect light having all wavelengths in the visible region. In the case of absorbing, the shielding layer is a layer having a black color whereas in the case of reflecting, the shielding layer is a layer having a white color. Therefore, there is the case where the problem arises that the attached information cannot be embedded in any of the region where the visible image is formed. Moreover, when the attached information is visually shielded with the shielding layer having a white color, it is necessary to pad the attached information between the layer on which the visible image is formed and the surface of an image output medium. The problem probably arises that no attached information can be newly added after the above shielding layer is formed.
On the other hand, in the technologies described in each of JP-A Nos. 6-171198 and 6-122266, nothing is defined concerning the absorptivity of the dye which can absorb or reflect infrared rays in the visible region. Therefore, like the above technologies described in JP-A No. 6-113115, the region and image for embedding the attached information are limited and no attached information can be newly added.
Moreover, the technologies described in JP-A No. 6-171198 are used to pad information made of an invisible image in the region where a visible image which is seen as a solid image by the eye is formed. There is therefore the disadvantage that the invisible image cannot be formed on a desired position on the surface of an image output medium irrespective of the position of the visible image formed on the surface of the image output medium.
In also the technologies described in JP-A No. 2001-265181, like the technologies described in the above publication, nothing is defined concerning the absorptivity of the toner forming the invisible image in the visible region and the same problem as above possibly arises.
Because, particularly, almost no studies as to recording materials such as a toner for forming an invisible image have been made in conventional technologies for forming invisible images as aforementioned, there has been the case where various problems arise which include for example, the problem that only an unsatisfactory accuracy is obtained when reading mechanically by infrared radiation as listed above and the problem that various restrictions are imposed when forming an invisible image.
On the other hand, in the conventional technologies described in each of JP-A Nos. 9-104857, 9-77507 and 7-53945 and concerning near-infrared light absorbing materials for forming invisible images, studies on the case of utilizing the near-infrared light absorbing materials as electrophotographic toners for forming invisible images are not made satisfactorily. It is therefore very difficult in practical use to form an invisible image with high accuracy while avoiding the occurrence of the aforementioned various problems listed above by using the technologies described in these publications.
In attached, it has been a common practice in recent secret documents and securities that a watermark image, a hologram image or the like is separately recorded as genuine recognition technologies. However, it is cited as a drawback that these measures are very expensive because specific paper and a specific recording method are used and also these measures need excessive labor for the management and protection of secrecy of the paper and recorders to be used.
Also, in technologies for preventing forgery and reproduction in which a specified pattern is formed on the surfaces of secret documents, securities and the like by using a conventional method of forming invisible image, an invisible image is recognized only by mechanical reading, whereby a real article can be discriminated from a forgery article. However, it cannot be, of course, even confirmed with the eye whether or not such an invisible image is present. Unlike, for example, a transparency formed on paper money, it has been impossible to obtain the effect of identifying the real and preventing a forgery simply with the eye.